In recent years, a water-cooling type heat sink is often used for effectively cooling a high-power laser diode. In order to increase the cooling efficiency, it is desirable to make the heat sink of a material with a high thermal conductivity. Furthermore, the material of the heat sink is preferably one having a coefficient of thermal expansion close to that of the laser diode. The reason for it is that if the laser diode and the heat sink thermally expand and contract with temperature change caused by heat generation during driving of the laser diode or by repetitive activation and deactivation of the laser diode, the difference between the coefficients of thermal expansion of the two materials will give rise to stress strain in the laser diode and result in degradation of characteristics of the laser diode. However, materials having a high thermal conductivity and a coefficient of thermal expansion nearly equal to that of the laser diode are expensive, and production cost will be very high if the whole heat sink is made of such a material.
Japanese Patent Application Laid-Open No. 2004-186527 discloses the heat sink that can effectively cool the laser diode and that can prevent the degradation of the laser diode characteristics due to the difference between the coefficients of thermal expansion. This heat sink has the structure in which three plates (upper plate, partition plate, and lower plate) are stacked. The upper plate is provided with a submount portion on which the laser diode is mounted, and only this submount portion is made of a material having the coefficient of thermal expansion nearly equal to that of the laser diode. In order to keep the production cost low, the portion (base portion) other than the submount portion in the upper plate, the partition plate, and the lower plate are made of a material consisting primarily of copper, which has the coefficient of thermal expansion higher than that of the laser diode.